By exposing the earth formation surrounding a well-bore to a gamma-ray (photon) source and observing the amount of radiation that returns or scatters, one can measure the average electronic density of the formation. This average or bulk density is in turn used to estimate the formation porosity, i.e., that volume of the rock formation that is potentially available to fluids or hydro-carbons. Accurate porosity estimates depend on the knowledge of the density of the underlying matrix rock (i.e. of the formation without the pore volume). As such one must identify also the rock type in the formation. This is done via a lithology or chemical composition measurement. The bulk density of rock matrix can be accurately parameterized in terms of its average atomic number.
Currently, most nuclear density tools rely on the measurement of scattered photons. Typically, photons travelling in the formation may scatter (Compton scattering) or be absorbed via the photo-electric effect. This approach is used both in “logging while drilling” and in wireline configurations for open-hole wells. In either case, the source of gamma-radiation employed is typically a high activity 137Cs cartridge that yields characteristic single-energy photons of 0.662 MeV.
The gamma-ray density measurement provides the best available measurement of formation porosity. Given their success, 137Cs sources are ubiquitously found in oil & gas fields around the world, and they have been used for decades in the E&P business. However, given the very high activity of the sources employed (1.5-2.0 Ci each), the continued use of 137Cs sources poses great safety and security risks, that service companies must manage with continued significant costs in terms of resources and energy.